1. Field of the Invention
The present invention relates to methods for rotationally orienting a downhole tool and, more particularly, but not by way of limitation, the invention relates to rotationally orienting such a downhole tool during directional drilling.
2. Setting of the Invention
In order to enhance the recovery of subterranean fluids, such as oil and gas, it is sometimes desirable to steer or direct a borehole towards a target that is not directly below the position of the well on the earth's surface. For example, in an oil producing formation which has little vertical depth and relatively greater horizontal extent with respect to the surface of the earth, a borehole which extends horizontally through the oil producing formation can produce more oil than one extending vertically through the formation.
In order to directionally drill a borehole horizontally, or at any selected angle, it is necessary to be able to steer the rotating drill bit. Numerous devices have been patented for this task. However, one such device is described in U.S. Pat. No. 4,699,224 which discloses one such apparatus and method which uses a flexible drillstring connected by a flexible joint to a reamer body and has a drill bit connected to an end thereof. An eccentric cylindrical collar is connected circumferentially at the downhole end of the flexible drillstring over the flexible joint connected to the reamer body. This causes the drill bit to pivot about the stabilizer in the opposite direction of the displacement created by the eccentric collar, thus the drill bit's trajectory can be altered or steered.
A borehole engaging mechanism is mounted to the outside surface of the thicker wall of the eccentric collar and digs into the borehole wall to prevent clockwise rotation of the eccentric collar. When the drillstring is rotated clockwise it rotates freely within the eccentric collar, but when it is rotated counterclockwise a springbiased latch mechanism latches the eccentric collar to the drillstring and causes the eccentric collar to rotate with the drillstring. This allows the eccentric collar to be rotationally reoriented with respect to the borehole.
Although the borehole engaging mechanism is designed to prevent the cylindrical eccentric collar from rotating with the drillstring during drilling, friction between the eccentric collar and the drillstring, together with downhole vibration and movement occurring during drilling, will tend to rotate the collar; thereby resulting in the need to reorient the eccentric collar periodically.
U.S. Pat. No. 4,948,925 describes a signalling device that can be used with the apparatus disclosed in U.S. Pat. No. 4,659,224 to generate a pressure pulse whenever the drillstring is radially, or rotationally oriented, at a preselected point on a collar or deflection tool. The signalling device is used to signal the orientation of the eccentric collar so that the borehole can be drilled in a desired direction. The normal operating procedure is to establish an initial orientation of a reference point near the lower end of the drillstring with a commercially available orientation technique, such as one using a mule shoe and either magnetic or gyroscopic surveying, as are well known in the art. The mule shoe is radially aligned with the latch on the drillstring at the time the drillstring is run into the borehole. After the survey is recorded, a reference mark is made on the drillstring or rotary table, to reference the position of the mule shoe and thus the latch. Since the rotational orientation of a collar recess with respect to the eccentricity is known, the rotational orientation of the eccentric collar with respect to the drillstring is known, and thus the reference mark on the drillstring can be observed to indicate the direction that the bit is being steered.
After a period of drilling (clockwise rotation), the drillstring can be raised slightly and the drillstring rotated counterclockwise to observe a pressure decrease when the orifice in the collar and orifice in the drillstring are aligned, i.e., when the latch is radially coincident with the recess. Since the latch is then aligned with the recess in the eccentric collar, the orientation of the reference mark at the surface can be interpreted to determine if the rotational orientation of the eccentric collar in the borehole has changed during the previous drilling period. Generally the orientation is observed while rotating both clockwise and counterclockwise to account for twist in the drillstring.
Problems with the previously described apparatus and procedure have occurred in that the procedure requires that drilling be interrupted to check its orientation. This interruption is required to raise the drill string, rotate the drillstring counterclockwise, observe the pressure pulse when the latch assembly opens, and determine whether the eccentric collar needs to be reoriented. These interruptions last for about three to eight minutes each and result in an inefficient drilling process, especially if it is found that no eccentric collar reorientation is needed. In some cases the verification process itself may disturb the orientation of the eccentric collar. Additionally, if it is found that the collar has moved, the amount of drilling that has occurred at unknown orientations and angles since the last verification of the proper positioning of the collar can not be determined.
Therefore, there is a need for an apparatus and method which will indicate the orientation of a downhole tool, such as an eccentric collar, without interrupting the drilling operation.